Swirl burners are devices that, by giving sufficiently strong swirl to an air flow, lead to the formation of a central reverse flow region (Central Recirculation Zone CRZ, vortex breakdown mechanism), which can be used for the stabilization of flames in gas turbine combustors.
Targeting best fuel-air premixing and low pressure drop is often a challenge.
Good fuel/air premixing must be in fact achieved in a mixing region before the CRZ where the flame is stabilized. This implies sufficiently high pressure losses in this region, i.e. the use of a swirler with high swirl number, allowing for high velocity tangential shearing in the fuel-air mixing section before vortex breakdown takes place.
High swirl number flows however give origin to excessive shearing at CRZ with significant increase of pressure losses just in this region. These pressure loss characteristics are shown in FIG. 1 from the Large Eddy Simulation of two different axial swirl burner arrangements with swirl numbers of 0.7 and 0.56. The burner arrangement 20 of FIG. 1(b) comprises an air tube 21 extending along burner axis 23 and opening at one end into combustion chamber 22. A central cylindrical bluff body 27 arranged concentrically within air tube 21 defines an annular channel for air and air/fuel flow to combustion chamber 22, resulting in central recirculation zone 26. Fuel is introduced into the air stream at a concentric swirl device 24 and mixed with the air in a subsequent mixing section 25.
The high swirl number variant (open squares in FIG. 1(a)) is characterized by a non-dimensional pressure loss Dp (measured in units given by the dynamic head in terms the bulk flow velocity in the mixing section) in the mixing section of approx. 2. This can ensure good fuel/air premixing but a rather large and not necessary pressure loss of approx. 8 at the CRZ.
The low swirl number variant (filled triangles in FIG. 1(a)) gives instead a pressure loss of approx. 0.7 in the mixing section which is not effective for fuel-air premixing and acceptable pressure drop of approx. 5.5 around the CRZ.
Thus, good air/fuel premixing and low pressure loss at the beginning of the CRZ is difficult to be put into practice at the same time with a single swirl device.
Document U.S. Pat. No. 6,438,961 B2 discloses a burner for use in a combustion system of a heavy-duty industrial gas turbine, which includes a fuel/air premixer having an air inlet, a fuel inlet, and an annular mixing passage. The fuel/air premixer mixes fuel and air into a uniform mixture for injection into a combustor reaction zone. The burner also includes an inlet flow conditioner disposed at the air inlet of the fuel/air premixer for controlling a radial and circumferential distribution of incoming air. The pattern of perforations in the inlet flow conditioner is designed such that a uniform air flow distribution is produced at the swirler inlet annulus in both the radial and circumference directions. The premixer includes a swozzle assembly having a series of preferably air foil shaped turning vanes that impart swirl to the airflow entering via the inlet flow conditioner. Each air foil contains internal fuel flow passages that introduce natural gas fuel into the air stream via fuel metering holes that pass through the walls of the air foil shaped turning vanes. By injecting fuel in this manner, an aerodynamically clean flow field is maintained throughout the premixer. By injecting fuel via two separate passages, the fuel/air mixture strength distribution can be controlled in the radial direction to obtain optimum radial concentration profiles for control of emissions, lean blow outs, and combustion driven dynamic pressure activity as machine and combustor load are varied.
Document US 2009/056336 A1 discloses a burner for use in a combustion system of an industrial gas turbine. The burner includes a fuel/air premixer including a splitter vane defining a first, radially inner passage and a second, radially outer passage, the first and second passages each having air flow turning vane portions which impart swirl to the combustion air passing through the premixer. The vane portions in each passage are commonly configured to impart a same swirl direction in each passage. A plurality of splitter vanes may be provided to define three or more annular passages in the premixer.
Document US 2010/293956 A discloses fuel nozzle auxiliary vane comprising a vane mountable base comprising a fuel inlet, wherein the vane mountable base is configured to mount to a surface of a main vane disposed in an airflow path of a fuel nozzle. The fuel nozzle auxiliary vane also includes a body extending from the vane mountable base, wherein the body comprises a fuel passage that turns from the fuel inlet to a fuel outlet, and the fuel outlet has a fuel outlet direction generally crosswise to a fuel inlet direction through the fuel inlet.
Document U.S. Pat. No. 7,137,258 B2 discloses a combustor including a center nozzle surrounded by a plurality of outer nozzles, the center nozzle and each of the outer nozzles having a fuel passage and an air passage, with a swirler surrounding the fuel passage and having a plurality of vanes projecting radially within the air passage, each vane having a trailing edge arranged at a swirl angle relative to a longitudinal axis of the nozzle, wherein the swirl angle for the swirler in the center nozzle is different than the swirl angle for the swirlers in the plurality of outer nozzles.
Document U.S. Pat. No. 7,578,130 B1 discloses methods and systems for combustion dynamics reduction. A combustion chamber may include a first premixer and a second premixer. Each premixer may include at least one fuel injector, at least one air inlet duct, and at least one vane pack for at least partially mixing the air from the air inlet duct or ducts and fuel from the fuel injector or injectors. Each vane pack may include a plurality of fuel orifices through which at least a portion of the fuel and at least a portion of the air may pass. The vane pack or packs of the first premixer may be positioned at a first axial position and the vane pack or packs of the second premixer may be positioned at a second axial position axially staggered with respect to the first axial position.
Document EP 2 685 164 A1 discloses an axial swirler for a gas turbine burner comprising a vane ring with a plurality of swirler vanes circumferentially distributed around a swirler axis, each of said swirler vanes comprising a trailing edge in order to achieve a controlled distribution of the exit flow velocity profile and/or the fuel equivalence ratio in the radial direction, said trailing edge being discontinuous with the trailing edge having a discontinuity at a predetermined radius.
Usually only one swirler is used for vortex breakdown and mixing. This is not optimal because good fuel/air premixing requires high swirl but this gives origin to too high pressure drop around the CRZ.